When a patient is examined for prescription eyeglasses, there are some parameters measured by the optometrist or ophthalmologist which are accurately determined through an examination. This examination is usually carried out using a phoropter, autorefractor, or aberrometer to measure the patient's eye aberrations (expressed in power, cylinder, and angle) as well as other means to determine eye related parameters and patient eyeglass preference parameters like frame size, frame style, pupil distances, pupil heights, vertex distance, pantoscopic tilt, wrap angle, tint preference, color preference, material preference, UV blocking preference, antireflection coating preference, and high order aberration correction preference. If the patient is being prescribed a progressive addition lens (PAL), which includes bifocal, multifocal and progressive addition lenses, then also measured is the patient's accommodation, which is the ability of the eyes to change focus between distant and near objects. If a patient has presbyopia, he is impaired in his ability to focus on close objects.
A PAL design corrects a patient's eyesight, but also helps with the accommodation from near to far by including two or more regions on the eyeglass lens which have different add power. The distance zone is typically on the top part of the lens, and corrects for some or all of the eye's aberrations when focusing at a distant object. There is also a reading zone on the lower part of the lens, usually toward the nose, and usually smaller than the distance zone, which has typically 1-3 diopters of added sphere power compared to the distance zone. There is a smooth transition between these zones in a PAL and an abrupt transition in a bifocal or multifocal lens. The transition zone has aberrations (blurring) and distortions (bending or waviness) except in a fairly narrow channel called a “corridor” connecting the zones. In this corridor the sphere power varies smoothly at intermediate values between the distance and reading power.
There are some subjective parameters associated with a PAL design, however, which do not come out of the eye examination, and instead are currently determined through the prescriber's and patient's judgment about what the patient might prefer for their lifestyle or uses for the glasses. In particular, there are two key parameters which are vital to a patient's happiness with a particular progressive addition lens design:                1. The tradeoff between the size of the distance vision field and the near vision field. If the distance field is small, then there are distortions at the edge of the lenses which cause the so-called “swim effect” and can be unsettling. If the reading field is small, then there is a very limited close field of view which can make reading difficult. If neither is small, then the distortions and aberrations in the transition regions become excessive and unpleasant.        2. Tradeoff between the length and width of the corridor. If the corridor is short, then the width of the corridor is small, and the lenses are ineffective at intermediate zones. If the corridor is long, then it is wider, but the reading zone may then be too small or moved out of the frames.        
Currently, manufacturers of PALs try to demonstrate these tradeoffs by showing the patient simulated images of what the patient might see when wearing glasses with PALs. However, these simulations cannot give the patient the experience of actually looking through the PALs and seeing the typical distortions. It is common practice to just manufacture a pair of glasses with Pals for a patient to try. Since a significant percentage of patients end up not liking those glasses, the current approach is expensive and wasteful.
What are needed are a device and a process for a patient to experience how a particular progressive addition lens will affect his vision, and for helping design progressive addition lenses to help establish the best tradeoff between the distance vision field and the near vision field to best satisfy particular patients.